docs: def com_google_fonts_check_family_underline_thickness(ttFonts):
    """Fonts have consistent underline thickness?"""
    underTs = {}
    underlineThickness = None
    failed = False
    for ttfont in ttFonts:
      fontname = ttfont.reader.file.name
      # stylename = style(fontname)
      ut = ttfont['post'].underlineThickness
      underTs[fontname] = ut
      if underlineThickness is None:
        underlineThickness = ut
      if ut != underlineThickness:
        failed = True
  
    if failed:
      msg = ("Thickness of the underline is not"
             " the same accross this family. In order to fix this,"
             " please make sure that the underlineThickness value"
             " is the same in the 'post' table of all of this family"
             " font files.\n"
             "Detected underlineThickness values are:\n")
      for style in underTs.keys():
        msg += "\t{}: {}\n".format(style, underTs[style])
      yield FAIL, msg
    else:
      yield PASS, "Fonts have consistent underline thickness."